Obturators have been used for many years on projectiles fired from conventional guns. An obturator protrudes slightly from the sidewall of the projectile and engages the interior of the gun barrel sidewall during firing to prevent blow-by by acting as a pressure seal. At the same time, the obturator engraves into the spiraled rifling grooves cut in the gun barrel sidewall thereby imparting rotation to the projectile as it leaves the gun barrel which enhances projectile stability during flight.
Copper bands have been used extensively as an obturator on many "dumb" projectiles because copper is soft enough to engrave into the lands during the ramming process and holds the projectile in position for firing. It also extrudes into the rifling grooves to function as a low pressure seal, but yet has sufficient strength to also serve as a high pressure seal during firing. Metal is typically very durable so that the copper obturator bands are not normally extensively misformed or damaged during routine handling of the dumb projectile.
Special problems exist for obturators designed for use on "smart" projectiles. Smart projectiles often contain an active guidance system and perhaps even communication equipment so that the flight of the projectile can be redirected subsequent to firing from the gun. Obturators for smart projectiles are most often of the despun type in which only a portion of the rotational velocity imposed on the obturator by contact with the rifling of the gun barrel is passed on to the projectile. The reason for this is two-fold; first it is necessary to protect the internal components of the projectile from the forces associated with a high rotational velocity, and second a fin stabilized projectile (as are most smart projectiles) does not require spin for stabilization. If the projectile includes deployable fins with active control surfaces that are used in the final stages of flight to direct the warhead to its target, projectile spin only complicates end maneuvers. Such despin-type obturators are normally ring shaped and include an inner surface that during the firing of the projectile fits against and rotates on a mating surface of the projectile sidewall. Thus, the slip coupling between the obturator and the projectile causes the projectile to rotate at a speed less than would be achieved by a direct coupled obturator but is still sufficient to impart some small rotational velocity to offset any thrust vector misalignments for rocket or ramjet boosted projectiles. Therefore, most despin-type obturators are fabricated from a relatively rigid nylon or plastic material which provides a more effective bearing surface for the despin coupling necessary for the smart projectiles; however, the rigid nylon or plastic obturators tend to break easily in cold weather handling, particularly if an edge or a corner surface is struck. In addition, this rigid surface does not engrave as well as copper in the gun rifling resulting in poor low pressure sealing.